Biology of the Basking Shark

The basking shark is the second largest fish in the world. A maximum length of 12.2m was recorded off Newfoundland but most sharks average around 7m in length. Maximum weight of 5-6 tonnes have been reported but around 2 tonnes is more typical. It is a harmless plankton-eater whose appearance on the surface in Irish coastal waters coincides with high densities of zooplankton, especially the copepod Calanus, whose dense swarms often turn the water red. Sharks filter this plankton by cruising slowly through this soup with their mouth open. Inside the mouth are gill-rakers made from keratin (like baleen from the great whales). About every minute or so the shark closes its mouth and 'gulps', contracting the gill-arches, where the mass of plankton and mucus passes into the stomach. It has been estimated that a 7m shark, with an open mouth of 0.5m2 and travelling at 2 knots will filter 1484 cubic metres of seawater every hour.

It has long been thought that basking sharks hibernated during the winter. This was thought to be a strategy to survive periods when plankton is very scarce and the squalene in the liver was a food deposit to sustain them during this period. This strategy was thought to lead to very slow growth and reproductive rates with a gestation period of 3.5 years suggested. As basking sharks are ovoviporous, bearing live young, and producing only around 6 young at a time. These life-history characteristics suggested a very low reproductive rate, making them vulnerable to over-exploitation. Recent research has suggested these estimates are too extreme but it highlights how little we know about this impressive species.

Results from recent satellite telemetry have shown long distance movements between Cornwall and Scotland with one shark travelling 1,878 km in 77 days across the Celtic Sea and up the western seaboard of Ireland before the tag stopped transmitting in the Minches in the outer Hebrides. These tags also recorded sharks at up to 850m off the edge of the continental shelf and were active throughout the winter when they tend to occur in deeper water but in similar habitats as the summer.